1. Technical Field
The present disclosure relates to a control apparatus for a hybrid vehicle which includes both an internal combustion engine and a motor as drive sources of the vehicle.
2. Description of Related Art
There has been known a hybrid vehicle (hereinafter also referred to as the “vehicle” for simplicity) which includes both an internal combustion engine (hereinafter also referred to as the “engine” for simplicity) and a motor as drive sources of the vehicle. Such a vehicle includes a storage battery which supplies electric power to the motor and which is charged by output of the engine.
In addition, when rotation of a wheel axle is transmitted to the motor, the motor generates electric power (i.e., an electric generator generates electric power), and the storage battery is charged by the electric power as well. Namely, the kinetic energy of the vehicle is converted to electrical energy, and the electrical energy is collected by the storage battery. This energy conversion is also called “regeneration.” When regeneration is performed, the motor generates a force for braking the vehicle (torque for decreasing the speed of the vehicle). The braking force is also called “regenerative braking force.”
The fuel efficiency (fuel consumption rate) of the vehicle can be improved by collecting, by means of regeneration during deceleration, a portion of energy consumed by the engine or the motor during acceleration or constant-speed travel of the vehicle, and storing the collected energy in the storage battery. During travel of the vehicle, the remaining capacity SOC (State of Charge) of the storage battery fluctuates.
Deterioration of the storage battery accelerates as a result of an increase in the remaining capacity SOC when the remaining capacity SOC is high and as a result of a decrease in the remaining capacity SOC when the remaining capacity SOC is low. Therefore, during travel of the vehicle, the control apparatus of the vehicle maintains the remaining capacity SOC at a level between a predetermined remaining capacity upper limit and a predetermined remaining capacity lower limit.
Incidentally, in the case where the vehicle travels in a downhill section, the vehicle continuously accelerates even when neither the engine nor the motor generates torque. Therefore, a driver of the vehicle removes his/her foot from the accelerator pedal and may press down on the brake pedal so as to request the vehicle to produce braking force. At that time, the vehicle restrains an increase in the vehicle speed by means of regenerative braking force and increases the remaining capacity SOC.
When the remaining capacity SOC increases; i.e., when the amount of electric power stored in the storage battery increases, the vehicle can travel over a longer distance by using the output of the motor only without operating the engine. Accordingly, if the remaining capacity SOC can be increased as much as possible within a range below the remaining capacity upper limit when the vehicle travels in a downhill section, the fuel efficiency of the vehicle can be improved further.
However, when the downhill section is long, the remaining capacity SOC reaches the remaining capacity upper limit, which makes it impossible to increase the remaining capacity SOC further. Accordingly, the greater the difference between the remaining capacity upper limit and the remaining capacity SOC at the start point of the downhill section, the greater the effect in improving fuel efficiency attained as a result of the travel in the downhill section.
In view of the foregoing, one conventional drive control apparatus (hereinafter also referred to as the “conventional apparatus”) raises the remaining capacity upper limit and lowers the remaining capacity lower limit when a travel route contains a downhill section having a predetermined height difference. In addition, the conventional apparatus puts higher priority to travel by means of the motor than to travel by means of the engine such that the remaining capacity SOC approaches the “lowered remaining capacity lower limit” to the greatest extent possible before the vehicle enters the downhill section (see, for example, Japanese Patent Application Laid-Open (kokai) No. 2005-160269).
Incidentally, in order to execute a control (downhill control) for increasing the remaining capacity SOC, while the vehicle is travelling in a downhill section, to thereby improve the fuel efficiency of the vehicle without fail, it is necessary to properly extract a downhill section (target-downhill-section) which is contained in a planned travel route and which is subjected to the downhill control. The conventional apparatus has extracted such a target-downhill-section by paying attention only to the above-mentioned predetermined height difference (height difference threshold). In other words, for extraction of such a target-downhill-section, the conventional apparatus did not take into consideration the air resistance acting on the vehicle.
More specifically, the air resistance acting on the vehicle is proportional to the square of the vehicle speed. Therefore, in the case where the vehicle speed during travel in a downhill section is high, an increase in the remaining capacity SOC is highly likely to become smaller as compared with the case where the vehicle speed during travel in that downhill section is low. More specifically, when the vehicle travels in a downhill section, the remaining capacity SOC increases as a result of conversion of the potential energy of the vehicle to kinetic energy and then to electrical energy. When the air resistance acting on the vehicle travelling in a downhill section increases, the loss produced at the time of conversion from the potential energy to the kinetic energy increases, and thus the amount of the obtained electrical energy (namely, the increase in the remaining capacity SOC) becomes smaller.
Therefore, when the conventional apparatus extracts a “downhill section in which the vehicle travels at high vehicle speed” as a target-downhill-section because the height difference of that downhill section is greater than the above-mentioned height difference threshold and executes the downhill control, there is very likely to arise a situation in which the remaining capacity SOC does not increase sufficiently before the vehicle ends the travel in that downhill section. As a result, the remaining capacity SOC may reach the remaining capacity lower limit, and forced charging (charging the storage battery by the output of the engine) may occur.
Meanwhile, in a downhill section in which the height difference is small but the vehicle travels at low vehicle speed, there is a possibility that electrical energy can be collected sufficiently. However, the conventional apparatus does not execute the downhill control for such a downhill section because the height difference is smaller than the height difference threshold. Accordingly, the conventional apparatus may fail to yield the fuel efficiency improving effect to a sufficient degree.